Manufacture of rubber bonded abrasive articles



Aug. 21, 1934 R. c. BENNER ET Al.

MANUFACTURE OF RUBBER BONDED ABRASIVE ARTICLES Filed Nov. 19, 1929 2 Sheets- Sheet Aug- 21, 1934- R. c. BENNER Er AL 1,970,834

MANUFACTURE OF RUBBER BONDED ABRASIVE ARTICLES Filed Nov. 19, 1929 2 Sheets-Sheet Patented Aug. '21, 1934 UNITED STATES PATENT OFFICE A MANUFACTURE OF RUBBER BONDED ABRA- SIVE ARTICLES Application November 19, 1929, Serial No. 408,312

9 Claims.

the surrounding structure, the harder is thegrade of the wheel. I n the case of rubber bonded wheels, or other abrasive articles, the grade is controlled in part by means of the compounding ingredients used in the manufacture thereof.

Referring more particularly to grinding Wheels, abrasive wheel manufacturers list about elevent different standardgrades of such wheels. Most of these grades are procurable inmany different grit sizes. In varying the grade`, therefore, by means of the relative amounts' of bond and abrasive grain, or by variation in the different amounts of the compounding ingredients, eleven diierent mixes are required to produce these eleven standard grades, and'these mixes must be provided in the various grit sizes. Such a method of varying the grade therefore necessitates a very large amount of labor and causes the waste of a great deal of valuable material Moreover, it necessitates the use of a large amount of equipment in producing and maintaining a large number of different abrasive mixes. i.'

We have been able to reduce the number of abrasivel mixes necessary for varying the gradesl of abrasive articles within Wide limits, accordconditions under which the abrasive article is` formed. According to the present invention, several different grades of wheels or other abrasive articles may be made from a. single mix without variation in the percentage of the ingredients used. This is done by selectively varying the period of application and the time of application ofa compacting pressure with reference to the plasticizing and vulcanization period of the rubber mix.

The accompanying drawing, Figure l, is a curve graphically indicating the variation in the grade of the article produced according to the time over a given range at which a heavy comi pacting pressure is applied to the mix. Figure 2 is a curve graphically illustrating the variation in grade secured byvfollowing a modification' of the process as described in connection with Figure 1.

According to the present invention, a vulcanizable mixture containing rubber and abrasive grain as essential constituents is employed. I The mix with which most uniformly satisfactvzoryv results have been obtained is preparedaccording to the disclosure of copendingfapplication of Porter and Nelson, Serialr No. 384,520, Iiled August 8, 1929. For the sake of completeness of description, this process .may be generally described. The mix is prepared from an aqueous dispersion of rubber. Preferably 'sheet rubber is dispersed in any suitable mann'er in a. suitable liquid medium, and a stock of such dispersed rubber is maintained for the manufacture of the abrasive wheels. A protective colloid, such as colloidal clay, casein or the like, is introduced into the dispersion, and a preservative such as formaldehyde and borax is also preferably used. Other materials which are used in the mix 'are magnesia` (magnesium oxide), lead acetate, abrasive grain and llers, sulphur, and Water. After the various constituents have been `separately Weighted or measured, the abrasive grains are first mixed with the llers, comprising one or more of the following substances: zinc oxide, carbon black, iron oxide, various forms of mineral rubber, etc.

This mixing process is carried on for a few minutes. The hardness of the wheel, as is Well known, will depend somewhat on the relative amount of and the character of the filler which is used. After the abrasive particles and filler have been thoroughly mixed for several minutes,-

water, lead acetate and a previously prepared mixture of rubber dispersion and sulphur are added in succession. These constituents are al1 mixed together for several minutes, after which the magnesia is added. The materials are then mixed again for a brief period, perhaps a minute. The mixture which has been obtained is then dried until it contains only about 7 percent moisture, 100 after which it is disintegrated by a process of hand raking, mechanical beating, or in any other suitable fashion. The matter of drying is quite important, as it has been found that there is a critical moisture content which should always obtain for the best results. If the mixture is too wet, the mix cannot be properly broken up or granulated. If the mix is too dry, it tends to become lumpy, which adversely affects the molding process.

. granules and grain clusters are of v very irregular shape and are easily deformable, as might be expected from the presence of rubber, so that the mix can be easily worked into the mold, and an even distribution of the particles throughout the mold secured. The dried disintegrated mass may be subjected to further drying, until the moisture content of the granules is in equilibrium with the normal atmosphere existing in the room. The dried material is then stored until needed.

While we have particularly described the process of preparing the rubber and abrasive mix disclosed in the said copending Porter and Nelson application, it will be understood that this isv merely a present preferred mix, because the material prepared in this way is very responsive to pressure and temperature control, but the in-v vention is applicable also to the formation of abrasive articles from mixes that are molded wet and subsequently dried. Batches of the vulcanizable rubber and abrasive mix are weighed out in amounts according to the requirements of the abrasive wheels or. other articles to be formed. These batches are then preheated Vto approximately 180 F. As thus preheated, the granules can be easily distributed in a heated mold in which the batch of material is placed after being preheated. The mold and abrasive mix are then heated to a temperature of about 240 F. at which temperature the mix becomes plastic. High pressures are applied for shortperiods at different times during the time when the mix is at the higher temperature of 287-300 F. and'according to the grade of the article-which it is desired to produce.

According to the preferred practice of. the invention, the mix in the molds is maintained under arelatively low pressure, from 300 to 500 pounds per square inch, during substantially the entire period'at which it is heated to 287-300 F., this period preferably being in the neighborhood of twenty minutes. At some time during this period, a high pressure is applied for a duration of about Grsdie reasure n ousandths of an S'lrlls] inchpenetrag High pressure applied at tion with a in pounds gouging tool per square at a given inch pressure A study of this table shows that the hardest abrasive articles were obtained by exerting 'the high pressure on the mixture for a given interval after about one-fifth of'thewhole period of heating had elapsed. The softest wheels were produced by exerting the high pressure for the same interval at the end of the twenty minute period. It will be noted that the high pressures are applied at a critical period between the plasticizing and vulcanizing conditions to obtain an article of the grade which is to be manufactured.,k During the first two minutes the mass is becoming plastic. As plasticity develops, the article is compacted readily. However, after a condition of plasticity is reached, vulcanization sets in, and as the extent of vulcanization increases, generally speaking, the effects of the high pressure application diminish. In the process of manufacture it is contemplated that the operator be given a table of pressures and of thel particular moment during this period at which the pressures are to be applied to aid him. in obtaining approximately the grade of wheel which it is desired to manufacture.

'Ihe effect of the high pressure on the resulting hardness is graphically illustrated in the accompanying drawings. These drawings, which are a curve plotted from the above table, show that when the high pressure is initially applied before the mass has become plastic, the resulting article will be relatively soft, but if the pressure is applied at the end of about four minutes, an article of maximum hardness will result. Generally speaking, if the high pressure is applied after a period of more than six minutes, the hardness of the article decreases with the length of time that the mass has been heated. Afterthe mass has been heated for about eighteen minutes and vulcanization has progressed considerably, the compacting effect of the high y pressure is considerably reduced and the resulting article is relatively soft. By way of explanation it should be stated that an article having the minimum penetration according to the above table has the maximum hardness.

After the grade of the wheel has been thus' determined by the application of the high pressure for a predetermined period, the pressure is reduced to a low value .which holds the shape of the article and maintains its predetermined density until partial vulcanization has proceeded sufliciently far to enable the article to hold its shape. AIt may then be removed from the mold and vulcanization completed at about 300 F. in an open oven, about twelve hours being required with the particular mix referred to for completion of vulcanization. Under these conditions no distortion, disruption due to the presence of bubbles, or subsequent swelling of the article is likely to occur.

From the foregoing it will be seen that if it is desired to produce a very dense wheel, the mixture is subjected to a low pressure of about 500 pounds per square inch for four minutes while it is heated with the press platens at a before partial vulcanization has progressed very far.

The advantages of our invention will be apparent. A large number of different grades of abrasive articles may be made from a single mix without variation in the chemicalor physical constitution of the mix. The granulated mix may be stored in a dry state, and when it is desired to make a wheel of a particular grade it can be done very quickly4 from this ready-touse composition. A good bond to grain adhesion is obtained, and by reason of this a large amount of work is secured from an abrasive grain before it becomes detached from the abrasive wheel. The grinding characteristics of the wheels are I particularly good because they are free cutting and cool cutting and in marked contrast to abrasive wheels made by the usual practice of milling the grains into abrasive sheets.

The invention constitutes a considerable departure from the practice which has heretofore prevailed of holding the mix under pressure for the entire period of vulcanization which requires several hours. By reason of this ,fact the molds are more quickly freed for use, and more articles can be produced with fewer presses and fewer molds. r

As a modication of our invention we contemplate that the grade of the wheels may be varied by the application of selective high pressures over the entire time for effecting the set cure. For instance instead of applying a low pressure over the major portion of the time required for set curing, and applying a high pres- Cil sure vtemporarily at a given stage in the process of set curing, a predetermined pressure may be continuously applied during this time. While the maximum hardness cannot be developed with the same maximum pressure by the process, very nearly similar results from a practical 'standpoint can be secured. The hardness of the articles will vary, according to the pressure applied, so that graded wheels can be obtained from a standard mix. This variation in the process has the advantage that less skill is required of the operator who is not called upon to apply the high pressure at a given time. The variation in the hardness according to the pressure applied over the entire period is illustrated by the following table where the period for the set cure is, in each case, twenty minutes;

Apparent density Pressure applied Relative hardness 24 2l (hardest) From this table it will readily be seen that the apparent density coincides with the increase in the grade or hardness of the article and the hardness increases with the increase of pressure.

We have found that this holds true with a variation in the amount of bond used.- as shown graphically by Figure 2, wherein curves are plotted based on three mixes containing different amounts of rubber and from mixes differing from the one used in making the objects from which the graph in Figure 1 were formed, the mixes of Figure 2 producing relatively softer articles, as will be clearly seen by comparison of the two tables.

It is primarily by reason of the fact that the mix is substantially dry when it is initially heated and subjected to pressure that it is practical to form the article and determine its grade in a relatively short set curing and thereafter vulcanize in an open oven without pressure and without subsequent distortion of the resulting product.

We claim:

1. In the process of curing rubber bonded abrasive articles, the steps which comprise heating a mixture of abrasive grain and vulcanizable rubber bond in a mold and applying pressures of different degrees during the time the mixture is being vulcanized in the mold.

2. In the process of curing rubber bonded abrasive articles, the steps which comprise heating a mixture of abrasive grain and vulcanizable rubber bond in a mold, applying moderate pressure to said mixture for part of the time it is being heated and applying heavy pressure to said mixture for the rest of the time the mixture is being vulcanized in the mold. Y

3. In the process of curing rubber bonded abrasive articles, the steps which comprise heating a mixture of abrasive grain and vulcanizable rubber bond in a mold, and alternately applying moderate and heavy pressures to said mixture during the time said mixture is beingvulcanized in the mold.

el. In the process of curing rubber bonded abrasive articles, the steps which comprise heating a mixture of abrasive grains and bond in a mold for approximately twenty minutes, and applying heavy pressure to the mixture for approximately two minutes of the time the mixture is being heated and moderate pressure for the remaining time.

5. In the process of curing rubber bonded abrasive articles, the steps which comprise giving a mixture of abrasive grains and vulcanizable rubber bond a set cure in a mold by applying heat and pressure simultaneously to said mixture and subsequently completing the cure by the application of heat alone to the mixture, the pressure applied during said set cure being moderate pressure for a part of the time and high pressure for the rest of the time.

6. In the process of curing rubber bonded abrasive articles, the steps which comprise giving a mixture of abrasive grains and rubber bond a set cure in a mold by applying heat and pressure to said mixture and subsequently completing the cure by the application of heat alone to the mixture, the pressure of said set cure being a heavy pressure for approximately one-tenth of the set curing time and a moderate pressure for the remaining time.

7. In the process of curing rubber bonded abrasive articles, the steps which comprise giving a mixture of abrasive grains and rubber bond a set cure in a mold by applying heat and pressure to said mixture and subsequently completing the cure by the application of heat alone to the mixture, the pressure of said set cure being approximately 3000 to 5090 pounds per square inch for approximately one-tenth of the set curing time and approximately 300 to 500 pounds per square inch for the remaining time.

8. In the process of curing rubber bonded abrasive articles, the steps which comprise heating a mixture of abrasive grains and rubber bond in a mold at low pressure for approximately four minutes, then heating and applying a high pressure to the mixture for approximately two minlnchfor approximately two minutes, and then heating and applying a pressure of 300 to 500 pounds per square inch for approximately fourutes, and then heating and applying a low pressure for approximately fourteen minutes.

9. In the process of curing rubber bonded abrateen minutes.

RAYMOND C. BENNER. CHARLES S. NELSON. GARNETT H. PORTER. 

